Automatic On-Drive Sync-Mark Search and Threshold Adjustment

ABSTRACT

A hard disk drive includes a processor to automatically adjust a threshold level for finding sync-marks. The processor determines all possible sync-mark patterns for a particular pattern length and analyzes each pattern with reference to real world data. The pattern with the largest distance gap is used. The threshold level is then adjusted dynamically to produce the lowest possible failure rate for the given pattern.

PRIORITY

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application Ser. No. 61/804515, filed Mar. 22, 2013,which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Threshold-based sync-mark detection is a commonly used scheme in readchannel systems. Detection is performed based on a Euclidean distancebetween a read-back signal and target sync-mark values. A threshold isset for sync-mark detection based on an anticipated distinction betweena sync-mark and a preamble. Performance of sync-mark detection isdetermined by the sync-mark pattern and threshold, and is calculatedbased on analytical models or synthetic waveforms. It is important forthe performance of a system to find a good sync-mark pattern andthreshold for a particular read channel. However, theoretically optimalsync-mark patterns and thresholds may not correspond to real harddrives.

Consequently, it would be advantageous if an apparatus existed that issuitable for dynamically adjusting the sync-mark threshold base on realdata.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a novel method andapparatus for dynamically adjusting the sync-mark threshold base on realdata.

At least one embodiment of the present invention includes a processor toautomatically adjust a threshold level for finding sync-marks. Theprocessor determines all possible sync-mark patterns for a particularpattern length and analyzes each pattern with reference to real worlddata. The pattern with the largest distance gap is used. The thresholdlevel is then adjusted dynamically to produce the lowest possiblefailure rate for the given pattern.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention claimed. The accompanyingdrawings, which are incorporated in and constitute a part of thespecification, illustrate an embodiment of the invention and togetherwith the general description, serve to explain the principles.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the present invention may be betterunderstood by those skilled in the art by reference to the accompanyingfigures in which:

FIG. 1 shows a representation of a signal from a data store;

FIG. 2 shows a method for finding a sync-mark and adjusting a thresholdfor finding a sync-mark;

FIG. 3 shows a block diagram of a drive system for implementingembodiments of the present invention;

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings. The scope of theinvention is limited only by the claims; numerous alternatives,modifications and equivalents are encompassed. For the purpose ofclarity, technical material that is known in the technical fieldsrelated to the embodiments has not been described in detail to avoidunnecessarily obscuring the description.

Referring to FIG. 1, a representation of a signal from a data store isshown. In one exemplary embodiment, a signal from a hard disc driveincludes a sync-mark 102. The sync-mark 102 defines a boundary between apreamble 100 and useful data 104. In other words, the sync-mark 102signals the beginning of useful data 104 in a read operation. In harddisk drive applications, a sync-mark 102 is a specially designedpattern, written to the media before the useful data 104. The sync-mark102 is later used for frame synchronization.

During read operations, a sync-mark 102 is detected based on a thresholddifference between a sync-mark pattern and a read-back signal. Thethreshold difference is defined by a Euclidean distance:

$D = {\sum\limits_{i = 0}^{L_{sm} - 1}\; {{{x\lbrack i\rbrack} - {{\hat{x}}_{sm}\lbrack i\rbrack}}}^{2}}$

In this equation, x is a pre-processed read-back digital signal sequenceand {circumflex over (x)}_(sm) is an ideal pre-processed digital signalsequence corresponding to the sync-mark 102 pattern. Ideal pre-processeddigital signal sequences are calculated by convolving a correspondingdata pattern with characteristics of the estimated read channel. In atleast one embodiment, the data pattern is a non-return-to-zero patterncommon in digital communication. A sync-mark 102 is found when thedistance D is less than some defined threshold.

For each read event, sync-mark 102 detection starts from the preamble100 and proceeds until the sync-mark 102 is detected. In a quarter-ratesystem where the sync-mark 102 has a length L_(sm), there areN=L_(sm)/4+1 possible data patterns to compare with the sync-mark 102.Such a system has N possible Euclidean distances. For example, where async-mark 102 is defined to have sixteen bits, there are five possibleEuclidean distances for a particular read channel and noise model.

In one exemplary embodiment of the present invention, where thedistribution of possible Euclidean distances is roughly Gaussian, async-mark 102 detection failure rate is estimated by the gap between theaverage sync-mark 102 distance D _(sm) (nearly zero) and the average 2Tpreamble signal distance D _(2T).

Sync-mark 102 detection failure occurs when a false sync-mark isidentified in a location other than the true sync-mark 102 location,such as in the preamble 100 (early detection) or no sync-mark 102 isdetected before or at the actual sync-mark 102 location (missdetection). The detection threshold adjusts the balance between the twofailure modes. A large distance gap D _(sm)− D _(2T) is preferablebecause it allows a wide threshold range, thereby reducing the failurerate. Thresholds and sync-mark 102 patterns are read channel specific.

In real hard disk drives, the theoretically ideal sync-mark pattern withmaximum Euclidean distance gap and optimal threshold may not providegood performance because real read channels are nonlinear anddata-dependent, which can be statistically different from the models,and the read channel varies from drive to drive depending on mediaconditions.

Referring to FIG. 2, a method for finding a sync-mark and adjusting athreshold for finding a sync-mark is shown. In at least one embodiment,the method for automatic on-drive sync-mark search and thresholdadjustment allows each individual hard disk drive to find a sync-markpattern and adjust a detection threshold based on actual data patterns.A processing element reads data input to detect 200 a sync-mark. In oneembodiment, the processing element begins with a predetermined sync-markpattern and detection threshold. In another embodiment, offlineprocessing 212 determines an initial sync-mark pattern and detectionthreshold based on theoretical parameters and data. The processingelement continues to process data input until a sync-mark is detected200.

When a processing element determines 202 that a sync-mark is detected200, the Euclidean distance of the detected sync-mark is recorded 204.Distance values are maintained in a buffer; in at least one embodimentthe buffer is of size N according to the bit-length of the sync-mark,which can keep track of the previous N distances. The N values stored inthe buffer correspond to the instantaneous distances between the Npossible signal sequences and the target sync-mark sequence: D_(2T), . .. , D _(sm). These values are used to compute 206 average distances.

In at least one embodiment of the present invention, a minimum gapbetween the detected distance and the sync-mark detection threshold isused to quantify the quality of a detection event. If the gap is smallerthan the minimum gap, the Euclidean distance will not be recorded evenif a sync-mark is detected 200.

In at least one embodiment of the present invention, the Euclideandistances are used for offline processing 212 to produce more precisesync-mark pattern selection and threshold adjustment.

Based on the instantaneous Euclidean distance in the buffer, a processorcomputes 206 an average Euclidean distances for preamble signals D_(2T)[n] and sync-mark signals D _(sm)[n]. In one embodiment, a two-taplow-pass filter is used to compute the average values:

D _(i) [n]=(1−α) D _(i) [n−1]+αD _(i) [n]

where α∈[0,1] is a small value depending on a channel variationparameter.

In at least one embodiment of the present invention, an updatedsync-mark is selected 208 based on the average Euclidean distances forpreamble signals D _(2T)[n] and sync-mark signals D _(sm)[n]. Aprocessor stores a sync-mark pattern and corresponding average Euclideandistance gap. In one embodiment, a default sync-mark pattern anddistance gap value is selected 208. For each new sync-mark pattern,after a predetermined number of data are processed, a processor readsthe average distances for preamble signals D _(2T)[n] and sync-marksignals D _(sm)[n] and computes a new distance gap:

G= D _(2T) [n]− D _(sm) [n]

The G value is compared with the current distance gap. If the newsync-mark pattern has a larger G than the current sync-mark pattern, thesync-mark pattern field and the distance gap field are updated. Finally,the sync-mark pattern with the maximum distance gap is stored and usedfor future sync-mark detection 200. In one embodiment of the presentinvention, all valid sync-mark patterns defined by the sync-mark lengthL_(sm) are processed and the one with the largest distance gap for theactual hard disk drive is selected 208.

For any given sync-mark pattern, a threshold is determined 210 accordingto the average distances for preamble signals D _(2T)[n] and sync-marksignals D _(sm)[n]. In at least one embodiment, a threshold D_(th) iscomputed by:

D _(th) [n]=β D _(2T) [n]+(1−β) D _(sm) [n]

where β∈[0,1] is a system parameter. The system parameter β adjusts thebalance between miss detection rate and early detection rate. Becausevalues of preamble signals D _(2T)[n] and sync-mark signals D _(sm)[n]are specific to the channel, the threshold D_(th)[n] is automaticallyadjusted 210 in real time. In one embodiment, the threshold D_(th)[n] isstored and used for future sync-mark detection 200.

A processor executing methods according to embodiments of the presentinvention searches for a sync-mark pattern with the largest distance gapfor a particular hard disk drive according to certain sync-markparameters. The processor also adjusts the detection threshold in realtime.

In another embodiment of the present invention, an offline processor isused for slower offline processing 212. The offline processor receivesthe instantaneous Euclidean distances from the distance buffer as theyare recorded 204, or at some later time. In at least one embodiment, async-mark failure rate is computed for any given threshold based on thereal Euclidean distance statistics. By analyzing all sync-mark patternsand thresholds within certain criteria, the sync-mark pattern andthreshold that achieves the smallest failure rate can be selected.

Referring to FIG. 3, a block diagram of a drive system for implementingembodiments of the present invention is shown. In at least oneembodiment of the present invention, a system such as a hard disk driveincludes a processor 300, memory 302 connected to the processor 300 andone or more data storage elements 304. The processor 300 is configuredto read data from the one or more data storage elements 304 and detect async-mark in the data stream.

In at least one embodiment of the present invention, the processor 300detects a sync-mark based on a threshold difference between an initialsync-mark pattern and a read-back signal. The threshold difference isdefined by a Euclidean distance:

$D = {\sum\limits_{i = 0}^{L_{sm} - 1}\; {{{x\lbrack i\rbrack} - {{\hat{x}}_{sm}\lbrack i\rbrack}}}^{2}}$

In this equation, x is a pre-processed read-back analog signal sequenceand {circumflex over (x)}_(sm) is an ideal pre-processed analog signalsequence corresponding to the sync-mark pattern. Ideal pre-processedanalog signal sequences are calculated by convolving a correspondingdata pattern with characteristics of the estimated read channel. In atleast one embodiment, the data pattern is a non-return-to-zero patterncommon in digital communication. The processor 300 detects a sync-markwhen the distance D is less than some defined threshold.

For each read event, the processor 300 begins sync-mark detection in thedata stream by reading a preamble and proceeding until the sync-mark isdetected. In a quarter-rate system where the sync-mark has a lengthL_(sm), there are

$N = {\frac{L_{sm}}{4} + 1}$

possible data patterns to compare with the sync-mark pattern. Such asystem has N possible Euclidean distances. For example, where async-mark is defined to have sixteen bits, there are five possibleEuclidean distances for a particular read channel and noise model.

In one exemplary embodiment of the present invention, where thedistribution of possible Euclidean distances is roughly Gaussian, async-mark detection failure rate is estimated by the gap between theaverage sync-mark distance D _(sm) (nearly zero) and the average 2Tpreamble signal distance D _(2T). A large distance gap D _(sm)− D2T ispreferable because it allows a wide threshold range. Thresholds andsync-mark patterns are read channel specific.

When the processor 300 determines that a sync-mark is detected, theEuclidean distance of the detected sync-mark is recorded in the memory302. The values stored in the memory 302 correspond to the instantaneousdistances between the possible signal sequences and the target sync-marksequence: D_(2T), . . . , D _(sm). The processor 300 uses these valuesto compute average distances.

In at least one embodiment of the present invention, the processor 300uses a minimum gap between the detected distance and the sync-markdetection threshold to quantify the quality of a detection event. If thegap is smaller than the minimum gap, the processor 300 does not recordthe Euclidean distance even if a sync-mark is detected 200.

Based on the instantaneous Euclidean distance in the memory 302, theprocessor 300 computes an average Euclidean distances for preamblesignals D _(2T)[n] and sync-mark signals D _(sm)[n]. In one embodiment,a two-tap low-pass filter is used to compute the average values:

D _(i) [n]=(1−α) D _(i) [n−1]+αD _(i) [n]

where α∈[0,1] is a small value depending on a channel variationparameter.

In at least one embodiment of the present invention, the processor 300selects an updated sync-mark based on the average Euclidean distancesfor preamble signals D _(2T)[n] and sync-mark signals D _(sm)[n]. Theprocessor 300 stores a sync-mark pattern and corresponding averageEuclidean distance gap in the memory 302. In one embodiment, theprocessor 300 selects a default sync-mark pattern and distance gapvalue. For each new sync-mark pattern, after a predetermined number ofdata are processed, the processor 300 reads the average distances forpreamble signals D _(2T)[n] and sync-mark signals D _(sm)[n] andcomputes a new distance gap:

G= D _(2T) [n]− D _(sm) [n]

The processor 300 compares the G value with the current distance gap. Ifthe new sync-mark pattern has a larger G than the current sync-markpattern, the processor 300 updates the sync-mark pattern field and thedistance gap field. Finally, the processor 300 stores the sync-markpattern with the maximum distance gap and uses that sync-mark patternfor future sync-mark detection. In one embodiment of the presentinvention, all valid sync-mark patterns defined by the sync-mark lengthL_(sm) are processed and the one with the largest distance gap for theactual hard disk drive is selected.

For any given sync-mark pattern, the processor 300 determines athreshold according to the average distances for preamble signals D_(2T)[n] and sync-mark signals D _(sm)[n]. In at least one embodiment,the processor 300 computes a threshold D_(th):

D _(th) [n]=β D _(2T) [n]+(1−β) D _(sm) [n]

where β∈[0,1] is a system parameter. The system parameter β adjusts thebalance between miss detection rate and early detection rate. Becausevalues of preamble signals D _(2T)[n] and sync-mark signals D _(sm)[n]are specific to the channel, the processor 300 automatically adjusts thethreshold D_(th)[n] in real time. In one embodiment, the processor 300stores the threshold D_(th)[n] in the memory 302 and uses the thresholdfor future sync-mark detection.

It is believed that the present invention and many of its attendantadvantages will be understood by the foregoing description ofembodiments of the present invention, and it will be apparent thatvarious changes may be made in the form, construction, and arrangementof the components thereof without departing from the scope and spirit ofthe invention or without sacrificing all of its material advantages. Theform herein before described being merely an explanatory embodimentthereof, it is the intention of the following claims to encompass andinclude such changes.

1. A hard disk drive comprising: a processor; memory connected to theprocessor, configured to store a reference sync-mark and a referencedistance threshold; a data storage element connected to the processor;and computer executable program code configured to execute on theprocessor, wherein the computer executable program code is configuredto: read a data stream from the data storage element; detect a firstsync-mark in the data stream, the first sync-mark having one or morecharacteristics substantially similar to one or more characteristics ofthe reference sync-mark based on the reference distance threshold;detect a second sync-mark in the data stream, the second sync-markhaving one or more characteristics substantially similar to one or morecharacteristics of the reference sync-mark based on the referencedistance threshold; update the reference distance threshold; determine aEuclidean distance associated with the first sync-mark; determine aEuclidean distance associated with the second sync-mark; determine anaverage Euclidean distance based on the first sync-mark and the secondsync-mark; determine an estimated early detection rate associated with anew distance threshold based on the average Euclidean distance; anddetermine an estimated miss detection rate associated with the newdistance threshold based on the average Euclidean distance.
 2. The harddisk drive of claim 1, wherein the computer executable program code isfurther configured to: store the Euclidean distance associated with thefirst sync-mark in the memory.
 3. The hard disk drive of claim 2,wherein the computer executable program code is further configured to:store the Euclidean distance associated with the second sync-mark in thememory.
 4. (canceled)
 5. (canceled)
 6. The hard disk drive of claim 1,wherein the computer executable program code is further configured to:select a new sync-mark based on the average Euclidean distance; andupdate the reference sync-mark to the new sync-mark.
 7. The hard diskdrive of claim 1, wherein the computer executable program code isfurther configured to: determine a Euclidean distance for two or morepotential sync-marks, each of the two or more potential sync-marksassociated with a sync-mark length; determine a distance gap for each ofthe two or more potential sync-marks; select a new sync-mark from thetwo or more potential sync-marks based on the distance gaps; and updatethe reference sync-mark to the new sync-mark.
 8. A method for detectingsync-marks in a data storage system comprising: reading a data streamfrom a data storage element; detecting a first sync-mark in the datastream, the first sync-mark having one or more characteristicssubstantially similar to one or more characteristics of a referencesync-mark based on a reference distance threshold; determining aEuclidean distance associated with the first sync-mark; detecting asecond sync-mark in the data stream, the second sync-mark having one ormore characteristics substantially similar to one or morecharacteristics of the reference sync-mark based on the referencedistance threshold; determining a Euclidean distance associated with thesecond sync-mark; determining an average Euclidean distance based on thefirst sync-mark and the second sync-mark; determining an estimated earlydetection rate associated with the new distance threshold based on theaverage Euclidean distance; determining an estimated miss detection rateassociated with the new distance threshold based on the averageEuclidean distance; calculating a new distance threshold; and updatingthe reference distance threshold to the new distance threshold. 9.(canceled)
 10. (canceled)
 11. (canceled)
 12. (canceled)
 13. The methodof claim 8, further comprising: selecting a new sync-mark based on theaverage Euclidean distance; and updating the reference sync-mark to thenew sync-mark.
 14. The method of claim 8, further comprising:determining a Euclidean distance for two or more potential sync-marks,each of the two or more potential sync-marks associated with a sync-marklength; determining a distance gap for each of the two or more potentialsync-marks; selecting a new sync-mark from the two or more potentialsync-marks based on the distance gaps; and updating the referencesync-mark to the new sync-mark.
 15. A computer apparatus comprising: aprocessor; memory connected to the processor; and computer executableprogram code configured to execute on the processor, wherein thecomputer executable program code is configured to: receive one or moreEuclidean distances corresponding to one or more sync-mark patterns;receive one or more Euclidean distances corresponding to one or moreread-back sequences; compute one or more distance gaps corresponding toat least one of the one or more sync-mark patterns and at least one ofthe one or more read-back sequences; determine an average sync-markEuclidean distance based on the one or more Euclidean distancescorresponding to one or more sync-mark patterns; determine an estimatedearly detection rate associated with a new distance threshold based onthe average sync-mark Euclidean distance; determine an estimated missdetection rate associated with the new distance threshold based on theaverage sync-mark Euclidean distance; and select a sync-mark pattern foruse by a hard disk drive.
 16. (canceled)
 17. (canceled)
 18. Theapparatus of claim 15, wherein the computer executable program code isfurther configured to determine an average read-back Euclidean distancebased on the one or more Euclidean distances corresponding to one ormore read-back sequences.
 19. The apparatus of claim 18, wherein thecomputer executable program code is further configured to: determine anestimated early detection rate associated with a new distance thresholdbased on the average sync-mark Euclidean distance and average read-backEuclidean distance; and determine an estimated miss detection rateassociated with the new distance threshold based on the averagesync-mark Euclidean distance and average read-back Euclidean distance.20. The apparatus of claim 15, wherein the computer executable programcode is further configured to: select a new sync-mark based on theaverage sync-mark Euclidean distance; and update the reference sync-markto the new sync-mark.